1. Technical Field
The invention relates to an imaging and display system for maintaining situational awareness when entering a dust cloud during a helicopter landing or takeoff.
2. Description of the Prior Art
Nature of the Problem
Helicopters frequently need to land in certain parts of the world where there is very fine sand and dust. Under a variety of circumstances determined by soil content and moisture, atmospheric conditions, and flight profile, the descending helicopters aerosolize this sand and dust with the downwash air stream from their rotor blades. Fine-grained dust and sand can stay aloft for up to 30 minutes after a landing. The resulting dust clouds create very dangerous low/zero visibility conditions at an extremely mission-critical time. This has resulted in the inability of the crew to execute their mission, damage to equipment, injury, and death.
There are two types of particle movement. FIG. 1 depicts a brownout generated during landing, where particles are moved outward by downwash 10, causing a sensation among the occupants of the helicopter 12 that the helicopter is moving backwards (away from the flow). A more serious effect is that particles are picked up by the vortices 11 swirling around the rotor blade tips of the helicopter 12, which causes them to travel upward in a toroidal cloud, surrounding the rotor and drawing an obscuring curtain around the aircraft.
In a brownout condition, the rotor wash circulates the dust particles upward, causing the pilot to lose visual reference during a critical phase of flight. Dry snow conditions can cause a similar experience, called whiteout. The pilot experiences spatial disorientation and is unable to perceive the helicopter's motion accurately across the surface. This can result in a number of dangerous situations:
If the helicopter drifts laterally during landing, the landing gear can be caught on an obstruction on the ground, leading to dynamic rollover.
The dust obscures objects and personnel on the ground, raising the possibility of a collision.
If the pilot experiences disorientation on landing and attempts a go-around, he will have no references as to drift or direction of movement, and will have to transition to instruments very close to the ground.
On takeoff, the moving dust can result in spatial disorientation, and the pilot may tilt the helicopter just before takeoff, introducing the chance of dynamic rollover; or the pilot may inadvertently let the aircraft drift immediately after takeoff. The obscuration also makes it hard to see tall obstacles on the takeoff path.
The obscuration becomes acute when the helicopter's airspeed falls below effective translational lift (ETL), generally 10-20 knots, below which airspeed the helicopter begins to fly in its own recirculating downwash. Flying into a headwind delays this transition so that it occurs at a lower groundspeed. For example, if a helicopter hovers in a 20 knot headwind, it is flying above ETL even though its ground speed is zero.
Pilots can alleviate brownout through several techniques, all of which have disadvantages:
Where the landing area is a smooth, unobstructed hard surface, the pilot can make a running landing. The helicopter is landed with sufficient forward speed to remain above ETL. This is more safely done with wheeled helicopters. However, brownout-producing surfaces are typically covered with soft sand or dust and are therefore not hard enough for a safe rollout. There is a risk of hitting a buried obstruction or sand pocket while rolling forward, thus decelerating the helicopter too quickly and causing it to tip forward. Nosewheel gear configurations, such as the MH-53, are less robust for running landings than tailwheel configurations such as the UH-60.
Where brownout conditions are not too severe, the pilot can set up a stable approach and limit his control inputs as the dust kicks up. However, there remains a danger that the obscuration is more serious than expected. The pilot typically does not become aware of this until landing is imminent. Also, undetected drift can develop at the last minute.
On takeoff, pilots are trained to lift off vertically until the helicopter clears the dust cloud, and then to translate forward and to climb out. This is not always possible where the helicopter is power-limited, since it takes more power to climb out of ground effect while airspeed is below ETL than it does to stay in ground effect until ETL.
According to the US Central Command, helicopter brownout incidents in the US military alone cost $100 million per year. However, this figure was exceeded within the first few weeks of the Iraq invasion, when at least five helicopters, including three AH-64 Apaches, were destroyed in brownout conditions. In 2005, incidents included the loss of two CH-47 Chinooks with 18 troops.
Previous Approaches
Technological solutions have been proposed to deal with brownout:
Autopilot/autolanding—Several systems now in use employ inertial navigation and/or Doppler radar to judge helicopter movement over the ground and either inform the pilot of drift or automatically fly the helicopter to a hover or to landing. These systems are adequate for stopping drift, but they do not enable the pilot to have unobstructed visual contact with the landing area and any potential obstacles.
Millimeter Wave Radar—A short wave radar system scans the area in front of the helicopter for terrain and obstacles. This system lacks adequate resolution and is generally too expensive to fit to all but a small number of specialized helicopters.
Laser Radar—In operation, this is broadly similar to millimeter wave radar but with better resolution and increased cost. As with millimeter wave radar, color representations must be artificially generated, decreasing a pilot's comprehension of his surroundings.
Rotor design—Several types of helicopters have less of a brownout problem than others of comparable weight due to their rotor or fuselage design. For example, the BERP (British experimental rotor program) blade tip as used on the Westland Lynx seems to reduce tip vortices and thus dust entrainment. The rotor blades on the seven-bladed Sikorsky CH-53E have non-lifting extensions at their roots, which creates an almost dust-free bubble around the cockpit. However, it is not practical to redesign and rebuild the thousands of helicopters now in service.